There are several tens of millions of residences in the United States (and many scores of millions of residences throughout the world) that have fireplaces for auxiliary heating and for enjoyment. Unfortunately, a conventional fireplace is grossly inefficient in that it is a source of a very large heat loss by way of natural draft up the chimney flue. About the only solutions proposed and adopted to any extent to improve efficiency are special fireplace installations that provide for controlled convection of heat recovered from the walls of the fireplace and, in some cases, the chimney to the living space by way of ducts and chambers built around the fireplace and chimney to receive heat and conduct it to the living space. Such installations are somewhat successful in increasing the amount of heat delivered to the living space, but do not solve the basic problem of eliminating heat loss due to chimney draft. It has also been proposed that some heat loss through chimney draft can be eliminated by providing some of the air that supports draft in the chimney from outside the living space, but that solution is only partially effective inasmuch as it is necessary for some air from the living space to go up the chimney in an open fireplace in order to prevent smoke from intruding into the living space. Accordingly, the suggestion of providing outside air to support combustion in the fireplace and chimney draft probably has as many disadvantages as advantages.
Within the last three or four years, the costs of residential heating have multiplied, and supplies of fuel oil and natural gas, the predominant sources of energy for generating heat for residential heating, have become increasingly uncertain. The tremendous increase in cost and the uncertainty of supply have created a great demand for efficient and economical auxiliary sources of heat. Though many persons are relying more on their fireplaces as such a source, the low efficiency of the conventional fireplace makes it a far from adequate solution to the problem.